Magnetic material provided with separator coating



Jan. 27, 1959 M. P. GETTING, JR I 2,

MAGNETIC MATERIAL PROVIDED WITH SEPARATOR COATING Filed June 29, 1956 IRubber Doctor Pol/s Ferrous Metal Evapofating Oven Insulating FilmComprising Carboxymeihy/ Hydroxyethy/ Ce/lu/ose and Refractory SubstanceFerrous Metal iii '2 JAM/hm Want gwbbim jrl.

United States I Patent MAGNETIC MATERIAL PROVIDED WITH SEPARATOR COATINGMilan P. Getting, Jr., Pittsburgh, Pa., assignor to Allis- ChalmersManufacturing Company, Milwaukee, Wis.

Application June 29, 1956, Serial No. 594,707

3 Claims. (Cl. 117-127) This invention relates in general to magneticsheet material employed in the manufacture of electrical ap-.

paratus such as transformers, and in particular to magnetic sheetmaterial provided with a coating of a refractory substance bonded to thematerial by a heat decomposable binder. The improved coated magneticmaterial may be used advantageously in manufacturing magnetic cores whenthe magnetic material must undergo various slitting, winding and formingoperations subsequent to being coated and prior to receiving a hightemperature anneal.

Electrical steel is sometimes supplied to the transformer manufacturerin a semifinished state in the form of large sheets or rolls. After thematerial is processed to a size convenient for the core manufacturingprocess, it is given a high temperature anneal which results in crystalgrowth in the preferred direction, further purification of the steel,and relief of any strains set up in the material during themanufacturing process. During the high temperature anneal the materialis usually in the form of a rolled strip or in the form of stackedlaminations depending on whether the steel is to be used in woundtransformer cores or in stacked cores. In either case, the magneticmaterial is placed in the annealing furnace and heated in a multilayeredassembly with relatively large surface contact between the layers. Dueto the plasticity of the steel at the elevated temperatures associatedwith this anneal there is a tendency for the associated layers in theassembly to stick together and possibly even weld to one another. It,therefore, becomes necessary to have a refractory substance betweenadjacent layers during the high temperature anneal to avoid thissticking.

One refractory substance or separator which is in common use, is asubstance consisting essentially of oxides of calcium and magnesium,although other substances have also been employed.

Various arrangements have been suggested in the prior art for applying arefractory separator coating to the magnetic material. In onesuggestedarrangement, the refractory substance which is a powder like substanceis applied to the magnetic material in a water slurry and then passedthrough an oven to evaporate the water. Because of the powdery nature ofthe refractory substance after the water has evaporated, it has atendency to rub off quite easily during subsequent handling operationsso that very little of the refractory substance is present during thehigh temperature anneal. It therefore becomes necessary with thisarrangement to apply the coating immediately prior to the annealingoperation which is a decided disadvantage. A further disadvantage isthat in being rubbed off during the handling operation unpleasant andunhealthy working conditions are created.

The prior art has also suggested providing magnetic material with aseparator coating comprising a refractory substance suspended in a heatdecomposable binder such as an alkyd resin or cellulose acetate insuitable organic solvents. When the solvent evaporates, the resinoussolvents.

lCC

component of the solution functions to bind the refractory substance tothe steel. Whileunagnetic material provided with this type of coatinghas several advantages over magnetic material provided 'with a coatingwhich is merely dusted on or applied in a water spray, it also has someserious disadvantages. The main disadvantage is tlhatthe cost on acommercial scale is relatively high because of the need of expensiveorganic Also, the use of organic solvents creates a serious fire hazardwhich is a decided disadvantage.

A third arrangement suggested in the prior art and similar to that justdescribed employs a coating including a small amount of methyl celluloseas the heat decomp'osable binder for the refractory substance. The costof the methyl cellulose binder is substantially less than the celluloseacetate or alkyd resin binder. However, when magnetic material, providedwith a coating including methyl cellulose as the binder for therefractory substance, is slit into strips and these strips wound andformed into the shape they will take as a finished transformer core, aconsiderable amount of the refractory substance is brushed off.

According to the present invention, magnetic material is provided with aseparator coating comprising a refractory substance in a finely dividedform bonded to the steel by a small amount of carboxymethyl hydroxyethylcellulose. It has been found that such a coating adheres to themagneticmaterial during subsequent slitting, winding, and formingoperations considerably better than any other coating suggested by theprior art.

It is therefore an object of the present invention to' provide magneticmaterials having an improved separator coating.

Objects and advantages other than those mentioned abovewill be apparentfrom the following description.

Fig. 1 is a schematic view of apparatus suitable for carrying out theinvention; and

Fig. 2 is an enlarged cross section through a sheet of material preparedin accordance with the invention.

In accordance with the present invention magnetic material in themicrocrystalline state is coated with a slurry containing oarboxylmethylhydroxyethyl cellulose, any known suitable refractory substance, andwater. Some suitable refractory substances are lime, dolomite, magnesiumoxide, and silica, and various combinations of the above.

This slurry may be prepared by adding pounds of refractory substance,preferably a mixture of calcium oxide and magnesium oxide, 325 mesh, and16.7 pounds of carboxymethyl hydroxyethyl cellulose to a suflicientamount of water to form a slurry. Carboxyrnethyl hydroxyethyl cellulosesold by the Hercules Powder Cornp-any under the name CMHEC has beenfound quite satisfactory. These ingredients are then ground together ina Wet ball mill for several hours until the refractory particles arethoroughly dispersed. It has been found that a suitable slurry isobtained when the specific gravity of the solution is substantially 1.07and its viscosity is 18.5 seconds in a No. 2 Zahn cup. The resultingsolution may then be applied to magnetic material as the first step inhandling the semiprocessed steel in a manner such as shown in Fig. 1.For example, if the semiprocessed steel is obtained in large rolls thesolution is applied to the steel as it is unrolled. Any suitable means Amay be employed for applying the coating to the material such as rubberdoctor rolls or brushes. After being coated, the magnetic material ispassed through an oven which causes the water in the solution toevaporate without harming the coating. The resulting coating shown inFig. 2 adheres to the magnetic material so that it may be freely handledin all types of slitting, Winding and forming operations which occurbefore the final anneal 3 without any substantial amount of therefractory substance rubbing 01?.

While only one embodiment of the present invention has been described itWill be apparent to those skilled in the art that other modificationsmay be employed without departing from the spirit of'the invention orfrom the scope of the appended claims.

What is claimed is:

1. Magnetic sheet material having thereon a separator coating comprisinga finely divided refractory oxide bonded to said material bycarboxymethyl hydroxycthyl cellulose adhesive binder.

2. A continuous silicon steel strip having thereon an adherent coatingcomprising a refractory substance consisting essentially of calcium andmagnesium oxide in a finely divided form and a small amount ofcarboxymethyl hydroxyethyl cellulose binder binding the refractorysubstance in intimate contact With the surface of said strip 4 toprevent said refractory substance from being subsequently rubbed as assaid strip is mechanically processe prior to annealing. '1

3. A strip of steel in the microcrystalline state provided with acoating comprising a refractory substance in finelydivided form bondedto the surface of said material by carboxymethyl hydrox'yethylcellulose.

References Cited in the file of this patent UNITED STATES PATENTS1,789,477 Roseby l Ian. 20, 1931 1,940,707 Browne Dec. 26, 19332,426,445 Frisch et a1. Aug. 26, 1947 2,515,788 Morrill July 18, 19502,618,632 Klug Nov. 18, 1952 2,640,791 Barber June 2, 1953 2,641,556Robinson June 9, 1953 2,739,085 McBride Mar. 20, 1956

1. MAGNETIC SHEET MATERIAL HAVING THEREON A SEPARATOR COATING COMPRISINGA FINELY DIVIDED REFRACTORY OXIDE BONDED TO SAID MATERIAL BYCARBOXYMETHYL HYDROXYETHYL CELLULOSE ADHESIVE BINDER.